1. Field of the Invention
This invention relates to methods and apparatus for enhancing the operation of optical devices which involve stimulated Brillouin scattering gain media, and more particularly to increasing the output energy of stimulated Raman scattering devices and increasing the conjugation fidelity of phase conjugate mirrors (PCM).
2. Description of the Related Art
Several optical devices are known which involve stimulated Brillouin scattering (SBS). For purposes of the present invention, devices employing stimulated Raman scattering (SRS) and phase conjugation are of greatest interest. The SRS phenomenon is described in, among other places, N. Bloembergen, "The Stimulated Raman Effect", American Journal of Physics, Vol. 35, November 1967, pp. 989-1023. A review of various applications for optical phase conjugation is presented by Giuliano in Physics Today, "Applications of Optical Phase Conjugation", April 1981, pages 27-35. A general review of the phase conjugation field is given in A. Yariv, IEEE, J. Quantum Electronics QE14, 650 (1978).
These devices rely upon the formation of optical "gratings" (i.e., periodic spatial variations in the optical properties) in a nonlinear optical medium. The Raman effect may be described as the scattering of light from matter, such as a gas, liquid or solid, with a shift in wavelength from that of the usually monochromatic incident radiation. The internal degrees of freedom (for example, electronic, vibrational, or rotational) of atoms or molecules of the medium couple with the incident radiation such that spatial variations within the medium result in a scattering of the incident radiation. This follows from the fact that the optical properties of the molecules vary with the excitation of their internal degrees of freedom.
Phase conjugation is an optical phenomenon that has attracted considerable attention in recent years. Several different ways of producing phase conjugated beams have been discussed in the literature, including four-wave mixing, SBS, SRS, three-wave mixing, photon echo devices and self-pumped PCMs. Basically, a PCM produces a retroreflection of an incident beam, with the phase of the reflected wavefront reversed with respect to that of the incident beam. PCMs can be provided either with external pumping beams, as in four-wave mixer devices, or as "self-pumped" devices in which external pumping beams are eliminated. Of the self-pumped PCMs, those employing SBS are generally used in connection with high power pulsed laser beams.
Serious problems have been identified for both SRS and SBS devices. In Raman devices, many gaseous media that could yield desirable wavelengths cannot be used because SBS is produced to a degree that swamps and suppresses the Raman process. In principle, SBS might be suppressed in Raman-active media by mode-locking the pump laser. However, this approach has not been demonstrated and would require a substantial redesign of the pump laser; it could also eliminate the SBS phase conjugation which often serves an essential function in the pump laser. In addition, it would increase the system complexity and the risk of optical damage.
With PCMs it has been difficult to maintain a good fidelity of the phase conjugated beam to the input beam at higher operating energies. Ideally, the phase conjugated beam should have a wavefront which mirrors that of the input beam but with a phase reversal. However, it has been found that as the input beam energy and intensity go up, the wavefront of the reflected beam increasingly diverges from that of the input beam.
The effect of the stimulation medium upon various stimulated phenomena has been a subject of investigation. Experiments on the correctness of the hydrodynamic equations in describing the behavior of binary gas mixtures under conditions of varying partial pressures, total pressures and stimulation frequency were related in W. H. Lowdermilk and N. Bloembergen, "Stimulated Concentration Scattering in the Binary-Gas Mixtures Xe-He and SF.sub.6 -He", Physical Review, Vol. 5, No. 3, March 1972, pp. 1423-1443. In particular, this article reported what was believed to be the first unambiguous demonstration of stimulated concentration scattering (SCS), and the conditions under which SCS would not be dominated by SBS. It purported to confirm the theory that the most favorable condition for SCS occurs in a mixture of gases in which the two constituent molecules have a large difference in mass and optical polarizability. The threshold of SBS was found to increase by mixing gases with a large difference of component masses. Experiments were conducted with various mixtures of SF.sub.6 and He and of Xe and He. As He was added, the maximum stimulated gain was found to decrease rather rapidly. For relative He concentrations less than 60%, the frequency shift of backscattered light was characteristic of SBS. Above 60% He concentration, a rapid decrease in the frequency shift took place that was characteristic of SCS. The complete coupled mode gain, the Brillouin gain, the concentration gain and the electrocaloric thermal gain were plotted as functions of He concentration in a He-Xe mixture. The uncoupled concentration gain was found to become approximately equal to the Brillouin gain at a relative concentration of 90% He. This work, however, did not propose any solutions to the SRS suppression or phase conjugate fidelity problems mentioned above.